NEXAFS Investigations of C60 / 3d -Transition Metal Interfaces

Physical properties of organic-metal contacts are subjects of interest in organic electronics and spintronics. With a better understanding on the electronic structures of the interface of metallic electrode and organic spacer, scientists hope to find ways to improve the carrier injection and detection efficiency so that hybrid devices with performance comparable to inorganic ones can be realized. In this study, we employ the near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to investigate the electronic structure of C60/3d -transition material interfaces [1]. Three series of bilayers with each series consist of a 0.36 nm-thick metal film (Fe, Co, or Ni) deposited on a set of C60 bottom layers (2 - 40 nm) were fabricated in-situ to allow systematic investigations on the metal contact and the thickness dependence of organics spacer. According to the Carbon K-edge NEXAFS spectra (see Fig. 1), we found clear evidence of orbital hybridization between C60 and the transition metals, the LUMO+1 and LUMO+2 show notable variations. In addition, the L3 -edge intensities of transition metal showed an unexpected C60 thickness dependence (see Fig. 2). Model analysis suggests that such thickness dependence is consistent with the scenario of metal atoms/clusters diffusing into organic layers. Satellite peaks are observed in the NEXAFS spectra of Fe/Co/Ni, and the deposition rate is modified to see how cluster size affects the spectra. Finally, detailed spectral analysis reveals subtle differences regarding the diffusion of Fe, Co, and Ni atoms/clusters within C60 film.